Fiberoptic telecommunications are continually subject to demands for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM). With a DWDM system many different and separate data streams may concurrently exist in a single optical fiber. Each data stream represents a different channel within the optical fiber, where each channel exists at a different channel wavelength. The modulated output beam of a laser operating at the desired channel wavelength creates the data stream. Multiple lasers are used to create multiple data streams, and these data streams are combined onto a single fiber for transmission in their respective channels.
The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers.
With the requirement for multiple tightly spaced channels, stable control over both the laser source and output frequency is important to system effectiveness. The lasers used in DWDM systems typically have been based on distributed feedback (DFB) lasers operating with a tuning etalon that defines the ITU wavelength grid. Due to manufacturing as well as performance limitations, DFB lasers are used as single channel lasers, or as lasers limited to tuning among a small number of adjacent channels. As a result, DWDM applications would require multiple different DFB lasers each at a different channel wavelength.
Continuously tunable external cavity lasers have been developed to overcome the limitations of DFB lasers. These lasers have a laser source and end mirror that define an external cavity used for wavelength tuning. For example, a tuning element within the external cavity, such as, an etalon device, is mounted to a support that is fixed to a platform extending between the laser source and end mirror. Controlling the temperature of the platform tunes the laser by altering the optical path length of the external cavity. Separate tuning between grid wavelengths may also be achieved by separately tuning the tuning element.
While such lasers have been used with some success, continuously tunable telecommunication lasers do present some design considerations. These lasers can be somewhat expensive and time consuming to manufacture. In particular, the tuning assemblies used in many continuously tunable lasers result from a lengthy sequential build-up process. During this process, certain components can only be fabricated after other components have been completed. Furthermore, once fabricated, the tuning speed of continuously tunable lasers is limited by the type of tuning method used in the assembly. More specifically, many current tuning methods require adjustment of multiple interdependent systems, which adds to control complexity and prolonged tuning times.